Combustion of coal, oil, industrial waste, domestic waste, peat, biomass, etc. produces flue gases that contain dust particles, often referred to as fly ash. Emission of dust particles to ambient air needs to be kept at a low level and therefore a filter of the electrostatic precipitator (ESP) type is often used for collecting dust particles from the flue gas before the flue gas is emitted to the ambient air. ESP's, which are known from, among other documents, U.S. Pat. No. 4,502,872, are provided with discharge electrodes and collecting electrode plates. The discharge electrodes charge dust particles which are then collected at the collecting electrode plates. The collecting electrode plates are occasionally rapped to make the collected dust release from the plates and fall down into a hopper from which the dust may be transported to landfill, processing etc. The cleaned gas is emitted to ambient air via a stack.
An ESP has a casing which encloses the discharge electrodes and the collecting electrodes and functions as a flue gas duct through which the flue gas flows from a flue gas inlet, past the discharge and collecting electrodes, and to a flue gas outlet. The ESP may contain, inside the casing, several independent units, also called fields, coupled in series. An example of this can be found in WO 91/08837 describing three individual fields coupled in series. Further each of such fields may be divided into several parallel units, which are often referred to as cells or bus-sections. Each such bus-section may be controlled, as regards rapping, power, etc, independently of the other bus-sections.
With more stringent demands for very low dust particle emissions from the ESP's it has become necessary to use a higher number of fields in series inside the casing of the ESP in order to obtain a very efficient removal of dust particles in the ESP. While an increased number of fields is effective to reduce the emission it also increases the investment and operating cost of the ESP.